1. Technical Field
The invention relates generally to Asynchronous Transfer Mode (ATM) networks wherein a source ATM device and a destination ATM device are connected by multiple intermediary network switching nodes, and relates in particular to a method for injecting control cells such as Operation And Maintenance (OAM) cells into the source-to-destination connection at an injection switching node and extracting the control cells from the connection at an extraction switching node.
2. Description of the Related Art
A connection is established in an ATM network from a source ATM device to a destination ATM device by multiple intermediary switching nodes. Incoming cells into a switching node are automatically routed to the next switching node of the connection. To this end, each cell includes an ATM header and a payload. An ATM protocol engine in the switching node identifies an incoming ATM cell utilizing a lookup table.
Upon a determination that an incoming cell is a valid cell (“valid” meaning belonging to an existing connection), the protocol engine performs traffic management functions related to traffic policing, congestion management, priority management, etc., and queues the cell in an appropriate queue. A scheduler utilizes priority-based scheduling procedures to select queues from which cells are transmitted. Upon selection by the scheduler, the cells are dequeued from their respective queues.
Prior to transmission of the cell from the switching node, the protocol engine adds routing labels to the cell including a switch routing label (SRL) and a protocol engine correlator (PEC). A cell formatted in accordance with such labeling by the protocol engine is processed within the switching node and will be referred to hereafter as a “labeled cell”.
The SRL contains either the identity of the destination blade explicitly, or a pointer to a translation table located in the switching device that contains the explicit destination blade identity. The PEC is a pointer utilized by the protocol engine of an output adapter of the switching node to identify the connection to which the cell belongs. The protocol engine in the output adapter receives the cell from a switching interface within the switching node. Similar to the input protocol engine, the processing performed by the output protocol engine on the cell includes: identifying the incoming cell by performing a lookup on the appended protocol engine correlator; performing traffic management functions; queueing the cell in the appropriate queues; dequeueing the cell in accordance with a scheduler. In addition, the output protocol engine removes the appended labels, swaps the ATM label, and transmits the cell on the connection to the appropriate destination ATM port(s).
Conventionally, transmitting control cells, such as Operation And Maintenance (OAM) cells, from an ATM switching node to another ATM switching node of the connection, requires transmission of control messages between the two switching nodes using specific hardware resources such a microprocessor dedicated to such a task in each switching node. However, such a solution is inefficient both in terms of cost and overhead devices inasmuch as it requires incorporation of microprocessors on the adapter cards of the switching nodes in addition to the Application Specific Integrated Circuit (ASIC) modules that are utilized to route non-control ATM cells.